1. Create every possible two-digit combination.
2. Find their sum.
3. Sum the original three digits.
4. Divide your answer from part 2 by your answer to part 3.

What number do you always get, and why?

If the 3 gigits are a,b and c all of which are different there are 3 combinations of 2 digits - ab; ac and bc.

Part2: The sum of the 2 digit combinations is 2a+2b+2c - equivalent to 2(a+b+c)

Part 3: Sum of the original digits is a+b+c

(Part 3)/(Part 2) = 2(a+b+c)/(a+b+c)

which always results in 2.